The IEEE (Institute of Electrical and Electronics Engineers) 802.16 standard provides technologies and protocols for supporting broadband wireless access. The IEEE 802.16 has been standardized since 1999, and the IEEE 802.16-2001 was approved in 2001. This is based on a single carrier physical layer, ‘WirelessMAN-SC’. Then, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ as well as ‘WirelessMAN-SC’ were further added to a physical layer of the IEEE 802.16a standard which was approved in 2003. The IEEE802.16-2004 standard revised after completion of the IEEE 802.16a standard was approved in 2004. The IEEE 802.16-2004/Cor1 was completed in 2005 in the form of ‘corrigendum’ so as to correct bugs and errors of the IEEE 802.16-2004 standard.
For reception and demodulation of data in a wireless communication system, a receiver and a transmitter have to be synchronized with each other. Especially, in a mobile communication system where a channel environment between a base station and a mobile station is continuously changed, synchronization has to be acquired through signaling between the BS and the MS for successful transmission and reception of data.
A communication channel between the BS and the MS largely consists of a downlink (DL) channel toward the MS from the BS, and an uplink (UL) channel toward the BS from the MS. In the DL corresponding to point-to-multipoint, a plurality of MSs are DL-synchronized with a data frame transmitted from the BS. For synchronization of the MSs, the BS may insert a preamble for synchronization into a part of a frame to be transmitted. Then, the MSs adjust DL-synchronization through the preamble. Here, the BS may use an additional synchronization channel.
In the UL, each MS has to transmit data to the BS in time and/or frequency domain allocated thereto so as to prevent interference between the MSs and so as to make the BS receive data. For UL synchronization, each MS is required to adjust synchronization through signaling with the BS, with consideration of a channel environment.
In the IEEE 802.16 standard, a signal transmitted and received between a BS and an MS for UL synchronization is defined as a ranging signal. Ranging is a series of procedures of controlling a transmission power and of adjusting time or frequency synchronization while the BS and the MS transmit and receive a ranging signal therebetween. That is, ranging may be referred to as a series of procedures for acquiring UL synchronization.
Initial ranging refers to a process of acquiring a precise timing offset between the MS and the BS, and a process of initially controlling a transmission power. Once power is turned on, the MS acquires DL synchronization from a DL preamble signal being received. Then, the MS performs initial ranging so as to adjust a timing offset and a transmission power. Differently from the initial ranging, periodic ranging refers to a process of periodically tracking a UL timing offset and a reception signal strength after the initial ranging.
Hereinafter, will be explained contention-based random access and non-contention-based random access by a mobile station (MS) for UL synchronization through a ranging channel.
In a 16m system which is undergoing a standardization process, contention-based random access and non-contention-based random access are performed for UL synchronization of an MS through a ranging channel. The contention-based random access is performed for initial ranging, periodic ranging and handover. On the other hand, the non-contention-based random access is performed for initial ranging and handover. In the 16m system, UL resources are partially allocated to the MS for random access through a ranging channel. Here, the MS may acquire a position and a size of the UL resources from control information transmitted from a BS. In the IEEE 802.16m system, a ranging channel is multiplexed through frequency division multiplex (FDM) with control channels and data transmission channels.
Hereinafter, the conventional 16m relay frame structure will be explained.
The conventional 16m relay frame structure is categorized into a uni-directional frame structure and a bi-directional frame structure according to an operation method of a relay station.
FIG. 1 illustrates an 802.16m uni-directional relay frame structure in accordance with the conventional art.
The conventional uni-directional relay frame structure and a function thereof will be explained with reference to FIG. 1. Referring to a vertical direction (i.e., column direction) of FIG. 1, the conventional 16m relay frame structure consists of a DL subframe and a UL subframe. Referring to a horizontal direction (i.e., row direction) of FIG. 1, the conventional 16m relay frame structure consists of a BS frame, an odd-hop RS frame and an even-hop RS frame.
A relay station (hereinafter, will be referred to as ‘RS’) is categorized into an odd-hop RS and an even-hop RS according to the number of hops with a base station.
A downlink of an odd-hop RS is divided into a 16m DL transmit zone and a 16m DL receive zone, and an uplink thereof is divided into a 16m UL receive zone and a 16m UL transmit zone.
(1-1) A 16m DL relay zone and a 16m UL relay zone of a BS                A 16m DL relay zone is a downlink zone of a BS, where the BS may transmit data to a 16m RS and a 16m MS.        A 16m UL relay zone is an uplink zone of a BS, where the BS may receive data from a 16m RS and a 16m MS.        
(1-2) A DL transmit zone/receive zone and a UL transmit zone/receive zone of a 16m RS                A 16m DL transmit zone is a downlink zone of a 16m RS, where the 16m RS may transmit data to a subordinate RS and a 16m mobile station (MS).        A 16m DL receive zone is a downlink zone of a 16m RS, where the 16m RS may receive data from a subordinate RS.        A 16m UL transmit zone is an uplink zone of a 16m RS, where the 16m RS may transmit data to a subordinate RS.        A 16m UL receive zone is an uplink zone of a 16m RS, where the 16m RS may receive data from a subordinate RS and a 16m MS.        
FIG. 2 illustrates an 802.16m bi-directional relay frame structure in accordance with the conventional art.
Referring to a vertical direction (i.e., column direction) of FIG. 2, the conventional 16m relay frame structure consists of a DL subframe and a UL subframe. Referring to a horizontal direction (i.e., row direction) of FIG. 2, the conventional 16m relay frame structure consists of a BS frame, an odd-hop RS frame and an even-hop RS frame.
(2-1) A transmit zone and a receive zone of a 16m RS                A bi-directional transmit zone is a transmit zone of a 16m RS, where the 16m RS may transmit data not only to a superordinate RS but also to a subordinate RS.        A bi-directional receive zone is a receive zone of a 16m RS, where the 16m RS may receive data not only from a superordinate RS but also from a subordinate RS.        
(2-2) A 16m DL access zone and a 16m UL access zone                A 16m DL access zone indicates a zone where a 16m BS or a 16m RS transmits data to a 16m MS.        A 16m UL access zone indicates a zone where a 16m BS or a 16m RS receives data from a 16m MS.        
The conventional 16m relay frame structure is defined as a transparent mode and a non-transparent mode according to functional characteristics. Hereinafter, the transparent mode and the non-transparent mode will be explained. In a transparent mode, a transparent RS does not transmit a preamble, an SFH, a BCH and a USCCH at a start portion of a frame. On the other hand, in a non-transparent mode, a non-transparent RS transmits a preamble, an SFH, a BCH and a USCCH at a start portion of a frame. That is, in the non-transparent mode, a control signal may be independently transmitted to a subordinate RS or an MS.
FIG. 3 illustrates a transparent relay mode frame structure in accordance with the conventional art, and FIG. 4 illustrates a uni-directional relay frame structure for coexistence with a transparent mode and a non-transparent mode in accordance with the conventional art. Referring to FIG. 4, a BS and a transparent RS are connected to each other, and a non-transparent RS performs a communication at a lower level of the transparent RS.
As aforementioned, the MS has to be directly synchronized with the BS or the RS through an uplink, or has to be indirectly UL-synchronized with the BS through the RS. Here, the MS may have a different UL transmission time according to links with the BS or the RS in various types of relay frame structures. Accordingly, required is a ranging channel with consideration of various types of relay frame structures.